FIG. 1 schematically illustrates an example of a digital multimedia network. A network 1 is used to exchange information between one or many broadcast operators, represented in boxes 2 and 3, and subscribers represented in boxes 4 to 6. The network may for example be a terrestrial television broadcast network in which the broadcast operators emit signals that are received by means of an aerial antenna by the subscriber. Another example of network may be a cable network, or a satellite transmission network.
Each broadcast operator may broadcast a number of digital multimedia services to the subscribers. The multimedia services may for example include television programmes, radio programmes, and interactive applications.
Each subscriber has a receiver decoder that is used to receive the multimedia services and in some cases to process the received multimedia services in order to render the multimedia services on a rendering device as appropriate. A television programme may for example be rendered on a television, and an interactive application may need to be executed by a processing system of the receiver decoder and generate interactivity with the subscriber.
Typically all digital multimedia services are received by all subscribers. More generally, all information broadcast by the broadcast operators over the network may potentially be received by all subscribers.
It is known to restrict access to multimedia services by attributing access rights to the subscribers. Hence a subscriber may only access such multimedia service for which he possesses corresponding access rights. A higher degree of restricted access to the multimedia services may be achieved by scrambling a service using a control word. In this case the subscriber may only access and unscramble the scrambled service if he possesses the control word.
Access right information may be broadcast to the subscribers 4-6 over the network 1. Typically the control word is broadcast to all subscribers using an Entitlement Control Message (ECM). However the control word may only be extracted from the ECM if the subscriber has access to an exploitation key that is periodically broadcast over the network in an Entitlement Management Message (EMM).
The publication of the international application WO01/17249A1 DEUTSCHE TELECOM AG concerns a method for releasing customer specific authorisations using security modules in conditional-access systems for chargeable devices. WO01/17249A1 describes to transmit EMMs to subscribers. A first possibility provides for transmitting an EMM using a Public Switched Telephone Network (PSTN), a mobile phone network such as a GSM network, or any other way of broadcast to transmit the EMM to a control device of a vendor. The vendor on turn uses the control device to transmit the EMM to a subscriber device in order to activate a subscription. The first possibility is said to be indirect because it requires an intervention of an intermediate between a provider and the subscriber, in this case the vendor. A second possibility provides for transmitting the EMM directly by broadcast to the subscriber device. A geographical localisation of the subscriber device may be effected by exploiting localisation data relative to a subscriber GSM device. A GSM network provider has the possibility to determine a geographical localisation of a subscriber's GSM device if this device is switched on. The knowledge of the geographical localisation of the subscriber device may allow to restrict the broadcast of the EMM to a restricted geographical localisation, if at all possible. The second possibility also mentions an embodiment according to which to which the subscriber first calls the EMM provider by using his GSM phone and subsequently obtains the EMM by broadcast.
WO01/17249A1 further describes the possibility to use the GSM phone network to transmit an EMM to a subscriber. However WO01/17249A1 advises not to use the GSM phone network for this purpose.
The publication Functional model of a conditional access system, EBU Technical review, Winter 1995, describes in paragraph 7.2.1 that it would be possible to establish a return path between a subscriber's decoder and a Subscriber Management System (SMS) using either the PSTN or a cable network, and to use this return path to transmit EMMs. One reason for using a return path would be to reduce broadcast bandwidth consumed by the broadcast of EMMs.